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Plant Cells and Tissues

Plants became truly land-dwelling with the advent of:

1. spores with durable, protective walls

2. thickened waxy cuticle over the epidermis

3. stomates that open and close

4. our old pal, lignin:

5. progression of alternation of generations so that the gametophyte is a small, ephemeral stage, and the sporophyte a large, persistent stage. (More on this later)

Once plants invaded terrestrial habitats, natural selection took over, and individuals with adaptive mutations radiated and changed to pioneer an entirely new world available to them. And that meant changing from unicellular, relatively undifferentiated ancestral algae into complex organisms with specialized cell types as well as simple and complex tissues and organs.


PLANT ORGANS

There are only three:

Plant organs are generally defined by the presence of more than one type of tissue. So before we embark on our study of plant organs, let's have a look at their components.


PLANT CELLS

One picture is worth a lot of words.

PLANT TISSUES

A tissue is defined as an aggregation of cells coordinated to perform a particular function or set of functions.

Tissues may be


MERISTEMS: Source of all other tissues

Meristems are regions of undifferentiated, embryonic cells. Initially, the cells are totipotent and can differentiate/mature into any other type of cell.

Recall the terminology for sequentially less versatile undifferentiated cells:

Meristems are present throughout the life of the plant, and are the source of seasonal new growth in both height (primary meristems) and girth (secondary meristems).
Meristematic cells are also found at the margins of such structures as growing leaves or flower petals. As they divide, they differentiate and lose their totipotency until the structure is fully grown and mature.

Height and Girth: Primary and Secondary Meristems

  • Primary meristems: located at the tips of roots and shoots. Responsible for increase in plant length. Apical meristems are located in two different areas of the stems and roots:

  • Secondary (a.k.a. lateral) meristems: located in the margins of the stem and root (vascular and cork cambium). Responsible for increase in girth. Note: Only plants with a vascular cambium can produce true, botanical WOOD, which is composed of concentric rings of xylem.

    Here are where you'll find the meristems...

  • Primary growth and primary structures arise from the apical/primary meristems.
  • Secondary growth and secondary structures arise from the lateral meristems.


    CELLS OF THE GROUND TISSUES


    CELLS OF THE VASCULAR TISSUES

  • ground cells (parenchyma, collenchyma, sclerenchyma)

    These form the structural support around the conducting cells.

  • PHLOEM

    More about companion cells and albuminous cells...

  • In angiosperms

  • In gymnosperms

    DERMAL TISSUES

    These are the "skin" of the plant. Epidermis is found in herbaceous plants, and in those that become woody, it is replaced by the periderm, which originates from the cork cambium.

  • EPIDERMIS
    This very complex tissue consists largely of flattened cells that lack chloroplasts. Specialized cells found in the epidermis include

  • PERIDERM
    This is a secondary epidermis, produced by the cork cambium. It consists of Directly beneath the periderm lies the phloem. The periderm and the phloem together comprise the BARK of a woody plant.


    The overall arrangement of tissues in a generalized stem cross-section can be seen here:

    And the progression of their development: